TWI Industrial Member Report Summary 560/1996

A C Elbro, T G Gooch and R J Pargeter

Sulphide stress corrosion cracking of ferritic steel may be promoted by a small fluctuation in the applied load.

Background

Transformable ferritic steels are susceptible to hydrogen-induced stress corrosion cracking (SCC) in a wide range of aqueous media. Hydrogen formed on the surface of the steel by a cathodic corrosion reaction can diffuse into the material, causing embrittlement, and, if the region is subject to tensile stress, cracking can result. In practice, the problem is especially severe in 'sour' media containing H2S; this species poisons the hydrogen atom recombination reaction, and thus induces pronounced hydrogen pickup by the material. Sensitivity to hydrogen embrittlement depends critically on the steel hardness, and a risk of cracking can arise even in fairly mild media, given sufficiently high hardness. Hence, welded joints may be particularly susceptible to service failure as a result of the local transformation during welding and consequent development of hardened structures. It has become common practice to seek to avoid the problem by imposing hardness limits, exemplified by the NACE criterion of 22HRC (equivalent to 248HV) hardness maximum. Restriction of peak hardness is normally applied both to parent steels and to welded joints.

However, such limits have been derived from field experience and laboratory tests under static loading. In practice, most plant undergoes some load fluctuation during services and it is known that imposition of elastic or plastic strain can stimulate both anodic and cathodic corrosion reactions: this could increase the degree of hydrogen ingress and the risk of cracking. (1)

Objective

To examine the effect of a small load fluctuation on the sulphide stress corrosion cracking behaviour of transformable steel.